To introduce myself, I'm a headhunter who specialises in people who do maths to banks...
I'm with Andy here, from the info you're sharing you will be late 30s /early 40s before starting work in a bank which is a definite possibliity but a more than slightly risky bet with your career.
Is it your aim to change tracks in academia or work in a bank ? (either can be goals, you don't need to choose yet)
I am sadly aware of the terrible career situation for math/science PhDs even if academic recruitment is something I rarely do, so this advice is aimed at banking careers.
Firstly, doing a PhD purely for career advancement is rarely optimal and is often quite damaging, some managers I know would think less of you for having 2 PhDs. Also the bank job market 3-5 years from now could be almost anything from wonderful to terrible. I'm actually currently being paid to model the future pay levels of PhD quants and one mainstream scenario is that in nominal terms it stays constant for the next decade. There was a time when that wasn't scary but inflation looks like rising.
Another risk is that this may not be what you're good at, it does happen and I will share that you will end up doing more programming, you OK about that ?
I believe your optimum is in one of the following:
1: Spending $1 K on a bunch of books, (Wilmott, Shreve, etc) working through them in your spare time. Asking questions on various fora when you get stuck. A bit of work on your stats and programming is also necessary.
Pros: Really cheap, doesn't risk your current track or earnings, some hiring managers see this as the gold standard for applicants.
Cons: You don't get a certificate saying you can do stuff, you need to manage your study plan and be really disciplined.
2: A part time MFE or the CQF
Pros: Relatively cheap, no loss of earnings
Cons: You have to be self disciplined and part time MFEs take twice as long as full time but cost just as much.
3: Full time MFE
Pros: Nice certificate to say you've done it, more support in studying.
Cons: Fees plus loss of earnings, also it is harder to move directly back into academia.
4: Squatting: auditing finance courses at your current university but taking no exams
Pros: Really cheap, no impact upon your current career.
Cons: No certificate and a rather random set of skills gained, best tried in conjunction with option (1)